1. Field of the Invention
The present invention relates to ceramic electronic parts and method for producing them. The ceramic electronic parts have a terminal section which reduces stresses which would otherwise be applied to the ceramic electronic part as a result of repeated thermal expansion and contraction.
2. Description of the Related Art
When ceramic electronic parts such as monolithic ceramic capacitors are mounted on an aluminum substrate they tend to break due to differences between the rates of thermal expansion of the aluminum substrate and the ceramic capacitor itself. An aluminum substrate exhibits a high level of heat dissipation and will expand and contract during consecutive temperature cycles of rising and falling temperature to a greater degree than the ceramic capacitor itself. This results in repeated stresses being placed on the capacitor which often result in breakage.
This problem is exasperated in high-capacity lead-based monolithic ceramic capacitors required in the electric power market, which have relatively low bending strength.
A partial solution to this problem is disclosed in, for example, Japanese Patent Laid-Open No. 5-283280. The disclosed monolithic ceramic capacitor has two external electrodes. Each external electrode has two corners and is provided with a terminal which extends from the external electrode. The terminal is bent to extend along the external electrode to be close to the external electrode. The monolithic ceramic capacitor is mounted on the substrate by soldering. Because the terminal extends away from the ceramic capacitor, stresses caused by thermal expansion and shrinkage of the substrate are absorbed by the movement of the terminal and are not applied to the capacitor body. A gap of 0.5 to 1.0 mm is formed between a cap section and the terminal, and the terminal is pre-plated with solder to improve soldering characteristics.
The terminal, however, has the following problems. Since the terminal is bent to be close to the external electrode, the solder used for mounting the monolithic ceramic capacitor on the substrate will easily bridge the gap between the terminal and the external electrode. The solder bridge inhibits the movement of the terminal and the terminal no longer acts as a buffer to the thermal stress applied to the substrate, and thus the monolithic ceramic capacitor will break as a result of thermal shock.
A larger gap between the bent terminal section and the cap will prevent bridging of the solder into the gap. However, in such a case, the gap between the capacitor body and the wiring substrate increases when the capacitor is soldered onto the wiring substrate. The increased gap will decrease the shear strength of the terminal, a strength which endures against deformation. If a transverse force is applied to the capacitor body, the terminal will be easily deformed. The deformed terminal will come into contact with the wiring substrate or the other electronic parts, resulting in a change in electronic characteristics or short-circuiting.
It is an object of the present invention to provide a ceramic electronic part free from the above-mentioned disadvantages.
A ceramic electronic part in accordance with the present invention includes at least one chip ceramic electronic part body having terminal electrodes provided on opposite sides thereof. Terminals, each preferably composed of a metal plate bent into a U shape, are attached to the terminal electrodes. Particularly, the outer face of a first leg of each bent terminal faces and is attached to a terminal electrode, and a second leg is attached to a substrate. The outer face of the bent terminal is provided with a solder-philic surface (a surface which solder does adhere to), and the inner face of the bent terminal is provided with a solder-phobic surface (a surface which solder does not adhere to).
According to one preferred embodiment of the present invention, the metal plate originally has two surfaces having excellent soldering characteristics and a solder-phobic surface is formed by treatment of one of the two surfaces to cause it to have a poor affinity for solder. The solder-philic surface is the other surface of the metal plate itself. The treatment may include, inter alia, coating a resinous material onto the surface of the metal plate. The resinous material may be selected from the group consisting of nonconductive resins, rubbers and conductive resins. The solder-philic surface of the metal plate may be a surface having solder formed thereon.
In another embodiment of the present invention, the metal plate has two surfaces having poor soldering characteristics, and the solder-phobic surface is one surface itself of the metal plate. The solder-philic surface is formed by treatment of the other surface of the metal plate to cause it to have an affinity for solder. The treatment may include placing a coating of a solder-philic metal material onto the surface of the metal plate. The solder-philic metal material may be selected from the group consisting of Ag, Au and Pd.
If desired, at least a portion of one bent leg may come into contact with the other bent leg.
The ceramic electronic part in accordance with the present invention may include one or more ceramic electronic part bodies. The terminal is attached to terminal electrodes of these ceramic electronic part bodies.